Epitaxial thin films, once the nearly exclusive preserve of semiconductor technology, have been recently used to advantage in several other technologies.
High-temperature superconductors, such as YBa.sub.2 Cu.sub.3 O.sub.7-x (YBCO), exhibit the best and most reproducible superconductive behavior when they are epitaxially grown as singly crystalline thin films on a crystalline substrate. Most of these high-temperature ceramic superconductors have an anisotropic perovskite crystalline structure. For instance, YBCO has a- and b-axis lattice parameters of 0.382 and 0.388 nm but a c-axis lattice parameter of 1.168 nm. Inam et al. have disclosed in U.S. patent application Ser. No. 07/531,255, filed May 31, 1991, a superconducting Josephson device in which two YBCO electrodes sandwich an insulating or at most semiconducting or normally conducting layer of PrBa.sub.2 Cu.sub.3 O.sub.7-x. All three layers are epitaxially grown with a c-axis orientation; that is, the c-axis is perpendicular to the plane of the film.
I and my co-inventors have extended this approach to ferroelectric materials for use in memories. In U.S. Pat. Nos. 5,155,658 and 5,168,420 both incorporated herein by reference and commonly assigned with the present invention, I and my co-inventors describe a device in which two YBCO electrodes sandwich a ferroelectric layer of, for example, PbZr.sub.x Ti.sub.1-x O.sub.3, where 0&lt;x &lt;1 (PZT), which is only slightly distorted from a cubic lattice structure. All the three layers are epitaxially grown so as to be singly crystalline thin films. In one embodiment, the YBCO is c-axis oriented, that is, the c-axis is perpendicular to the film surface and the bottom YBCO layer is grown directly on a LaAlO.sub.3 substrate. It is not anticipated that such ferroelectric memories would operate at superconducting temperatures; instead, they rely on the normal conductivity of the YBCO electrodes.
Both superconductive devices and ferroelectric devices would enjoy greater commercial success if they could be integrated on a silicon substrate with silicon electronic circuitry. As a result, the technology has developed for epitaxially growing YBCO on Si. Fork et al. have disclosed such a technique in "High critical currents in strained epitaxial YBa.sub.2 Cu.sub.3 O.sub.7-.delta. on Si," Applied Physics Letters, volume 57, 1990, pp. 1161-1163. They first deposit a layer of yttria-stabilized zirconia ((Y.sub.2 O.sub.3).sub.x (Zr.sub.2 O.sub.3).sub.1-x, hereinafter YSZ) on a silicon substrate and then deposit the YBCO on the YSZ.
Although I suggested the structure Si/YSZ/YBCO/PZT/YBCO in my parent U.S. Patent Application, it is not completely satisfactory for many important applications. The electrodes should be structurally and electrically symmetric, and the upper YBCO electrode needs to be grown at a temperature too high for the underlying, already deposited PZT. Furthermore, several recently discovered high-temperature ceramic superconductors, such as BaKBiO.sub.3 do not have an anisotropic perovskite crystal structure, but instead have a cubic or nearly cubic crystal lattice structure. Also, electrodes for ferroelectric memories do not even need to be superconductive at any temperature but only need to show a reasonable conductivity. Recent work on singly crystalline ferroelectric memories have included conductive oxide electrodes composed of lanthanum strontium cobalate (La.sub.1-x Sr.sub.x CoO.sub.3, where 0&lt;x&lt;1, hereinafter LSCO), grown on SrTiO.sub.3. Cheung et al. discloses this structure in "Conductive and epitaxial LaSrCoO thin film grown by pulsed laser deposition as electrodes for ferroelectric PLZT devices," Abstracts: 4th International Symposium on Integrated Ferroelectrics, Monterey, Calif., March 1992 at p. 9 C. This material, which can be grown at a much lower temperature, is nominally cubic with crystal lattice parameters of 0.382.+-.0.01 nm. By nominally cubic is meant that the lattice parameters do not differ by more than 5% and the crystal lattice axes are perpendicular to within 3.degree.. Furthermore, a need exists to directly form monocrystalline layers of ferroelectric materials such as PZT or dielectric materials such as strontium titanate (SrTiO.sub.3) on Si substrates. My experience is that nominally cubic perovskites cannot be epitaxially grown with high quality on YSZ-buffered Si substrates.